Berthelsen, H., Hansen, L. T. 1999. The effect of hay on the behaviour of caged rabbits (Oryctolagus cuniculus). Animal Welfare 8, 149-157.

When hay was available [placed on top of cage], the [single-caged] rabbits ... performed significantly less bar gnawing and excessive grooming and were less restless. This suggests that rabbits kept in cages where hay is available are less stressed than those kept in cages where it is not. When kept in otherwise barren cages, rabbits interacted with the hay 16% of one-hour observation sessions.